Hydration of Portland Cement
Introduction
Portland cement is a hydraulic cement, hence it derives its strength from chemical reactions
between the cement and water. The process is known as hydration.
Cement consists of the following major compounds (see composition of cement):
Tricalcium silicate, C3S
Dicalcium silicate, C2S
Tricalcium aluminate, C3A
Tetracalcium aluminoferrite, C4AF
Gypsum, CSH2
Chemical reactions during hydration
When water is added to cement, the following series of reactions occur:
The tricalcium aluminate reacts with the gypsum in the presence of water to produce
ettringite and heat:
Tricalcium aluminate + gypsum + water ® ettringite + heat
C3A + 3CSH2 + 26H ® C6AS3H32, D H = 207 cal/g
Ettringite consists of long crystals that are only stable in a solution with gypsum. The
compound does not contribute to the strength of the cement glue.
The tricalcium silicate (alite) is hydrated to produce calcium silicate hydrates, lime and heat:
Tricalcium silicate + water ® calcium silicate hydrate + lime + heat
2C3S + 6H ® C3S2H3 + 3CH, D H = 120 cal/g
The CSH has a short-networked fiber structure which contributes greatly to the initial
strength of the cement glue.
Once all the gypsum is used up as per reaction (i), the ettringite becomes unstable and
reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate
crystals:
Tricalcium aluminate + ettringite + water ® monosulfate aluminate hydrate
2C3A + 3 C6AS3H32 + 22H ® 3C4ASH18,
The monosulfate crystals are only stable in a sulfate deficient solution. In the presence of
sulfates, the crystals resort back into ettringite, whose crystals are two-and-a-half times the
size of the monosulfate. It is this increase in size that causes cracking when cement is
subjected to sulfate attack.
The belite (dicalcium silicate) also hydrates to form calcium silicate hydrates and heat:
Dicalcium silicates + water ® calcium silicate hydrate + lime
C2S + 4H ® C3S2H3 + CH, D H = 62 cal/g
Like in reaction (ii), the calcium silicate hydrates contribute to the strength of the cement
paste. This reaction generates less heat and proceeds at a slower rate, meaning that the
contribution of C2S to the strength of the cement paste will be slow initially. This compound
is however responsible for the long-term strength of portland cement concrete.
The ferrite undergoes two progressive reactions with the gypsum:
in the first of the reactions, the ettringite reacts with the gypsum and water to form
ettringite, lime and alumina hydroxides, i.e.
Ferrite + gypsum + water ® ettringite + ferric aluminum hydroxide + lime
C4AF + 3CSH2 + 3H ® C6(A,F)S3H32 + (A,F)H3 + CH
the ferrite further reacts with the ettringite formed above to produce garnets, i.e.
Ferrite + ettringite + lime + water ® garnets
C4AF + C6(A,F)S3H32 + 2CH +23H ® 3C4(A,F)SH18 + (A,F)H3
The garnets only take up space and do not in any way contribute to the strength of the
cement paste.
The hardened cement paste
Hardened paste consists of the following:
Ettringite - 15 to 20%
Calcium silicate hydrates, CSH - 50 to 60%
Calcium hydroxide (lime) - 20 to 25%
Voids - 5 to 6% (in the form of capillary voids and entrapped and entrained air)
Conclusion
It can therefore be seen that each of the compounds in cement has a role to play in the
hydration process. By changing the proportion of each of the constituent compounds in the
cement (and other factors such as grain size), it is possible to make different types of
cement to suit several construction needs and environment.
References:
Sidney Mindess & J. Francis Young (1981): Concrete, Prentice-Hall, Inc., Englewood Cliffs, NJ,
pp. 671.
Steve Kosmatka & William Panarese (1988): Design and Control of Concrete Mixes, Portland
Cement Association, Skokie, Ill. pp. 205.
Michael Mamlouk & John Zaniewski (1999): Materials for Civil and Construction Engineers,
Addison Wesley Longman, Inc.,
